combined value to db-type coersion routines into single
`coerce-value-to-db-type` fn

db-objs makes use of this as does :clsql-helper-slot-coercer system
(which defines a (setf closer-mop:slot-value-using-class) method)
using the same coersion routine.

re AccelerationNet/clsql-helper#3
re fisxoj/clsql-helper
re Programming:#449 (1.75)